Hmmmmm, the title to this thread is "10 Major Ski SINS" and the bulk of what I see posted here seems to me as whining and drivel about bad behavior in the lodge or on the lift. Since the words "Ski SINS" are in the title, I will suggest my few that pertain to the actual activity:
1. Skiing in boots that have more than 250 ski days on them.
2. Skiing on skis that have not been tuned in 20 ski days.
3. Skiing with bindings that are so old that no ski shop will even touch them.
4. Skiing on slopes that are so overly steep for your ability that you have to side-slip down the headwall scraping the snow off said headwall (making you equal to a snowboarder in over their head scraping the snow clean while riding their heel edge down the headwall), or worse, freaking out and freezing in place on the headwall until professionals can show up and drag your sorry ass down in a sled.
5. Bringing "friends" skiing onto slopes that are too overly steep for their ability, and then abandoning them there.
6. Blasting into main slopes from perpendicular side trails, without first slowing and looking uphill before entering to avoid others already on the main trail and not expecting your sudden entrance.
7. Ducking a rope onto a closed trail, and then having to be rescued from it.
8. Stopping to rest in the middle of a tricky narrow area of a slope, where avoiding you will be difficult for skiers behind you and your position will compromise their path.
9. Having a collision with someone else (regardless of whose "fault"), and not remaining with them until help arrives or they are able to ski away under their own power.
10. Giving a friend you're skiing with a technique pointer, without first asking if they even want to hear your pointer.
I'm sure I missed some, but those 10 quickly came to mind...